WAN usually refers to the connection you get from your ISP (Globe, PLDT, Bayan, etc..)
Dual WAN, means, you can utilize 2 WAN in one network.
Naa to gi-post si moklo nga di na magrequire og router or server with para maka dual WAN.
Usually, in a dual wan setup, you'll need a Router with 2 WAN input, like this..

Then naa ka i-setup aron magamit nimo ang duha ka internet.
kung ari ka'g SERVER side mag dual WAN like Zentyal as suggested ni boyguapo, mag need ka og 3 ka LAN Card...
1st LAN card = WAN1
2nd LAN card = WAN2
3rd LAN card = to Switch
then imu pang i-setup pud ang imung server to receive two internet connection and serve the connection sa imu clients.